The present invention relates to an improvement of a binder for the formation of a coat layer which is used in a method of manufacturing a mold composed of the steps of forming a refractory coat layer on a mold pattern (hereinunder referred to as "pattern"), removing the pattern, and firing the refractory layer to obtain a desired mold.
The refractory coat layer is generally formed on the pattern as an accumulative refractory coat layer by repeating a process comprising the steps of coating the pattern with a liquid binder or a mixture slurry consisting of the liquid binder and a refractory powder, sanding the coat layer with refractory particles, and drying and hardening the coat layer until a desired thickness is obtained for an accumulative layer.
As the pattern used in the above-described process, a urea pattern, which is water-soluble, has both high strength and high dimensional stability and, in addition, is inexpensive, is desirable. As the binder used in the above-described process, a binder which does not attack a urea pattern when it comes into contact therewith is desired.
As binders obtained by improving binders such as a binder obtained by the hydrolysis of ethyl silicate with an acid and a binder obtained by adding amine to ethyl silicate, Japanese Patent Publication No. 22929/1979 discloses a binder obtained by mixing a polyethyl silicate, an amino group-containing organofunctional hydrophilic silicon compound such as .gamma.-aminopropylethoxysilane, and a non-polar solvent such as xylene in a specific ratio, and a binder obtained further mixing said binder with an alcoholate of titanium, zirconium, aluminum, etc. in a specific ratio.
A binder obtained by the hydrolysis of ethyl silicate with an acid is poor in preservation stability and, in addition, when it comes into contact with the surface of a water-soluble pattern such as a urea pattern, the binder dissolves the surface of the pattern and attacks it, thereby disadvantageously lowering the precision of the mold obtained by the above-described process.
On the other hand, a binder obtained by adding amine to ethyl silicate is good in preservation stability in a hermetic state, but when it comes into contact with the surface of a water-soluble pattern such as a urea pattern, the binder is also apt to attack the surface of the pattern due to the presence of amine in the binder. Furthermore, while a coat layer is formed on a pattern, this binder emits an odor of amine, thereby exerting a deleterious influence on the working atmosphere. In addition, the binder which has been preserved in a container is apt to increase in viscosity or gel while it is used in an open state.
The binder which is disclosed in Japanese Patent Publication No. 22929/1979 and which contains no metal alcoholate does not emit an odor of amine and has a considerably low degree of surface attack on a water-soluble pattern such as a urea pattern and a good preservation stability in a hermetic state, but since the binder in a container comes into contact with air in an open state when forming a mold, its properties are changed after about one-week use; for example, a considerable change in the viscosity is observed. The binder which is disclosed in Japanese Patent Publication No. 22929/1979 and which contains a metal alcoholate shows a considerably good but still insufficient stability while it is used in contact with air in an open state. Furthermore, since a change in properties is caused even during preservation in a hermetic state and the hardening speed thereby changes, it is difficult to set the hardening speed, which is an important factor in forming a mold.